Frequency modulation



y 15, 11947- R. SPRAGUE 2,423,952

FREQUENCY MODULATION Filed Dec. '12, 1941 K J/GIVAL GENERATOR I 2 If 150 i; 2% .L E I RAD/0 v rRA/vsm/rm? ROBERT M. SPRAGUE INVENTOR Patented July 15, 1947 FREQUENCY MODULATION Robert M. Sprague, Great Neck, N. Y., assignor to Press Wireless, Inc., Chicago, 111., a corporation of Delaware Application December 12, 1941, SerialNo. 422,613

9 Claims.

This invention relates to frequency modulators and more especially to such modulators for use in facsimile signalling systems and the like.

A principal object of the invention is to provide an improved arrangement for converting signals such as telegraph signals, facsimile signals and the like, into a frequency spectrum.

I have disclosed in my prior application Serial No. 301,563, filed October 27, 1939, an arrangement for producing a frequency modulated carrier in the audio frequency spectrum, which requires two high frequency oscillators of different frequencies which are mixed or heterodyned to produce the required audio frequency carrier. Frequency modulation of this audio frequency carrier is effected by changing the capacity across a tuned circuit of one of the high frequency oscillators by means of a specially connected control tube. In order to achieve the requisite stability in the audio frequency carrier with this prior arrangement it was found necessary, under certain circumstances, to have the oscillators housed in accurate heat-controlled compartments or ovens so that change in temperature of the surroundings would not affect the audio frequency waves.

Therefore, another object of the present invention is to overcome the above-noted and other disadvantages in frequency modulators by generating the audio frequency carrier directly in an audio frequency oscillator tube which has the requisite stability. 7 r 7 A-feature of the invention relates to an improved and stable audio frequency oscillator.

Another feature relates to an improved arrangement for frequency modulating an audio frequency oscillator. A further feature relates to an improved frequency modulation arrangement which is particularly well suited for facsimile transmission and the like.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which constitute an improved, simple and. stable audio frequency oscillator.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed description and the appended claims.

7 In the drawing which illustrates certain embodiments,

Fig. 1 is a generalized schematic diagram of an improved frequency modulator and an audio frequency oscillator according to the invention.

Fig. 2 is a. preferred modification of Fig. 1.

Referring to Fig. 1 of the drawing, numeral I represents any well-known signal transmitter for producing electrical variations of current, potential, resistance or impedance in accordance with a message or intelligence signal to be transmitted. This device may be a telefacsimile transmitting machine wherein the shade values of elemental areas of a subject are converted into electrical signals. For a detailed description of a typical transmitter of this type, reference may be had to Patent No. 2,209,719. In accordance with the present invention, the electrical signals from device I are applied to vary a pair of resistances or a pair of condensers in a novel form of stable audio frequency oscillator. Thus as shown in Fig. 1, the two resistances 2, 3, are of the variable type each being connected to the device I so as to be varied thereby in accordance with the amplitude of the electrical signals. Each of the devices 2 and 3 may be a grid-controlled electron tube whose plate resistance is arranged to be varied in accordance with the electrical signals from device I impressed on the grids of such tubes. Each of the resistances 2, 3, is associated with a corresponding condenser I, 5, to determine the frequency of the oscillations generated by tube 6.

Tube 6 is a grid-controlled vacuum tube having an electron-emitting cathode I, a control grid 8, and an anode 9. While the drawing shows a tube of the triode or single grid type, it will be understood that any other well-known grid-controlled electron tube of the multi-grid type may be employed. The positive potential for the'plate electrode of tube 6 is supplied through coupling resistor I 0 from a suitable source of D. C. potential, the positive terminal of which is designated by the numeral II. While the drawing shows a cathode I of the indirectly heated type, a filamentary cathode can be employed. A suitable grid bias resistor I2 is connected between the cathode and the grounded terminal of the plate supply so as to bias the grid 8 to the proper amount whereby tube 6 operates over the linear portion of its Eg--Ip characteristic curve.

Tube 6 is arranged to function as a generator of oscillations by feedback action between its grid and plate circuits, the frequency of the oscillations being determined by the resistors 2, 3, and the associated condensers 4, 5. The feedback path of tube 6-is controlled by the plate-to-cathode discharge path of another grid-controlled vacuum tube I3 which provides the necessary phase inversion of the voltages fed back to grid 8 to maintain tube 6 in an oscillating condition. The voltages developed across coupling resistor 10 are impressed through variable resistor 2 an condenser 4 on the control grid 14. The cathode I5 is connected to ground through grid-bias resistor l6 so that tube l3 operates on the linear portion of its Eg-Ip characteristic curve. The plate I! is connected through coupling resistor l8 to the positive plate supply terminal II. The potential variations of plate I! are fed back through condenser l9 to grid 8 in the proper phase to maintain tube 6 oscillating. The frequency of these oscillations is determined by the condensers 4, 5, and the particular setting of the variable resistors 2, 3, which setting is controlled by device I in any well-known manner.

I have found that by choosing the proper values for the electrical elements 2, 3, 4, 5, audio frequency oscillations of remarkable frequency stability are generated, this stability being such as to avoid the necessity of employing temperaturecontrolled ovens or the like for the tube 6 and associated elements. To shift the frequency of the oscillations, it is merely necessary to vary the value of the resistors 2, 3, simultaneously and approximately the same amount. if desired, the resistors 2, 3, can be fixed and the condensers 4, 5, can be varied under control of device 1, or both resistors and condensers can be varied. The signals from device I therefore, result in an audio frequency spectrum represented by corresponding voltages in the coupling resistor 13. These voltages are then applied through condenser Hi to a load resistor 29 which feeds the frequencymodulated waves to a suitable transmitter 2| which may be any well-known form of modulated high frequency transmitter such as a radio transmitter.

Referring to Fig. 2, there is shown a modification of Fig. 1 wherein the resistors 2 and 3 are replaced respectively by variable resistance gridcontrolled tubes 22, 23. The operation of Fig. 2 is substantially the same as that of Fig. 1, and the parts which are identical in the two figures bear the same designation numerals. The electrical signals from device I are applied over conductor 24 to the control grids 25, 2B of tube 22,

and also to control grids 21, 28 of tube 23. Preferably the grids 21 and 28 are connected to the adjustable arm 29 of potentiometer resistance 30 across which the electrical signals are applied. Preferably also, the tubes 22 and 23 are of the dual triode type wherein the two control grids are connected together as shown. The tubes 22 and 23 may each have a single electron-emitting cathode 3|, 32, cooperating with the associated grids and plates, these cathodes being connected to ground through suitable grid-bias resistors 33, 34. It will be understood of course that tubes 22 and 23 may have dual cathodes corresponding to the associated dual grids with the dual cathodes of each tube directly connected. The plates 35, 36, are connected in push-pull relation to the primary Winding 31 of an audio frequency transformer 38, the midpoint of this primary winding being connected to the positive terminal I l of the plate supply. The secondary transformer winding 39 is connected in series withthe frequency control condenser4 to the grid E4 of tube 13. Likewise, this secondary winding 39 is connected in series with condenser 4 and the secondary winding 40 of another audio frequency transformer 4|, this secondary being shunted by the frequency control condenser 5. The primary winding 42 is connected in push-pull relation to the, plates 43, 44 of tube 23, the midpoint of this primary winding being connected to the plate supply ll. secondaries 39 and 49 perform, in conjunction with the associated primary windings and tubes 22 and 23, the same function as 'plate-to-plate dynamic impedance of the respective tube. This impedance is reflected in the secondaries 39 and 49, the value of the reflected impedance or resistance depending upon the turns ratio of the said transformers. Thus the frequency of oscillation of tube 6 will then depend upon the elfective resistance reflected into the transformer windings 39 and 4!! under control of the respective tubes 22 and 23. As the plate resistance of these tubes is varied under control of device 1 the effective resistance (equivalent to the resistances 2 and 3 of Fig. 1) is varied, thereby changing the frequency of oscillation of the tube 6 the required amount. Since the respective plate impedances of the tubes 22 and 23 change approximately as the square of the voltage on their control grids, and since the frequency of the oscillator 3 varies inversely-as the variation of the equivalent effective resistances 39, 43 (Fig. 2) the frequency change of oscillator 3 is proportional to the voltage impressed on the grids of the tubes 22, 23, resulting in the desired linear relation between the signals from source I and the generated-oscillation developed across the load resistor 29.

Thepush-pull balanced relation of the plates of tubes 22 and 23 is necessary so that the modulating voltagesimpressed on the respective controlgrids will not appear in the output of the system. The amount of degeneration in the oscillator circuit changes slightly as the effective resistances across windings 39 and 43 is varied and this may result in changes .in amplitude of the oscillations. This effect can be overcome by adjusting the voltage on the grids of one control tube, e. g., tube 22, faster than the variation on the grids of the tube 23. This adjustment can be accomplished by adjusting the arm 29 of the input potentiometer. The arrangement of Fig. 2 in addition to providing the necessary frequency stability of the audio frequency oscillator enables instantaneous frequency shift without the generation of undesirable transients. This is particularly important in connection with the transmission of black and white subject matter such as printed matter and the like, since the scanning of sudden change from black to white has a tendency to produce a delay in the ordinary frequency modulation systems and the generation of undesirable transients. Preferably, the transformers 33 and 4! are well balanced with respect to capacity and inductance in order to achieve the maximum freedom from transients.

While certain specific embodiments have been disclosed herein, various changes and modifications may be made without departing from the spirit and scope of the invention. Thus, while in the drawing the variable resistance tubes 22 and 23 are shown as of the dual element type with the dual sets of elements contained in a single bulb, it will be' obvious to those familiar with the art that two separate triode tubes can be employed With the sets of triode electrodes connected in the same manner'as illustrated. Accordingly, in the attached'cl'aims, by the expression grid-controlled variable resistance tube is intended to be included arrangements where the sets of triode elements are arranged in one bulb or are arranged in separate bulbs.

What I claim is:

1. An audio frequency oscillation system comprising a pair of grid-controlled vacuum tubes, one of which is arranged to generate oscillations by feed-back action, a feed-back path for said oscillation tube including a reactance connecting the plate of said other tube to the grid of the oscillator tube, a path having a variable effective resistance connecting the plate of the oscillator tube to the grid of the other tube, another path having a variable effective resistance connected between the grid of said other tube and the cathode thereof, and signal-control means to vary the effective resistances of said paths and thereby to vary the frequency of said oscillations, the

cathodes of both said tubes having a common ground return.

2. An oscillation system comprising a grid-controlled oscillator tube, a feed-back circuit for said oscillator tube, said circuit including another grid-control1ed tube having its plate connected through a reactance to the grid of the oscillator tube, and a variable frequency-determining circuit for said oscillator tube, said circuit comprising a pair of effective variable resistors connected in series with a condenser between the plate and cathode of the oscillator tube, one of said variable effective resistors also being connected across the grid and cathode of said other tube, and signalcontrol means for varying said effective resistors to thereby vary the frequency of said oscillations.

3. A system according to claim 2 in which said variable resistors are arranged to be varied simultaneously approximately the same amount under control of a given signal.

4. An audio frequency oscillation system comprising a grid-controlled oscillator tube, a feedback circuit for said oscillator tube, said circuit including another grid-controlled tube having its plate connected through a reactance to the grid of the oscillator tube, a frequency-determining circuit for said oscillator tube comprising a pair of resistance-capacitance combinations, said combinations being connected in series between the plate and cathode of the oscillator tube, and one combination also being connected across the grid and cathode of said other tube, and signalcontrolled means to vary said effective resistances to thereby vary the oscillation frequency.

5. An audio frequency oscillation system comprising a pair of grid-controlled vacuum tubes, one of which is arranged to generate oscillations by feed-back action under control of the plate circuit variations of a feed-back control tube, a frequency-determining circuit for said oscillator tube including a first transformer and a second transformer, each transformer having a primary winding in said frequency-determining circuit, another pair of tubes of the variable resistance grid-controlled type, the transformers having their primary windings connected in series with a condenser acrossthe plate and cathode of the oscillator tube, the primary winding of the second transformer being connected across the grid and cathode of said feed-back control tube, and means to impress a signal to be transmitted upon the grids of said variable resistance tubes to vary their effective plate resistances and thereby to vary the frequency of said oscillations by varying the effective resistance in said frequency-determining circuit.

6. An audio frequency oscillation system according to claim 5 in which frequency adjusting condensers are connected across each of said transformer primary windings and the secondary windings of said transformers are connected respectively in the plate circuits of said variable resistance tubes.

7. A system according to claim 5 in which said variable resistance tubes are of the dual triode type with the two plates of each tube connected in balanced relation to the secondary windings of the associated one of said transformers.

8. A system according to claim 5 in which the means for impressing signals on the grids of said variable resistance tubes includes a potentiometer across which the signals to be transmitted are impressed and having a variable contact arm which is connected to the grid of one of said variable resistance tubes and another point on said potentiometer being connected to the grid of said other variable resistance tubes.

9. An oscillation system comprising a grid-controlled oscillator tube, a feedback path for said tube including another grid-controlled tube whose plate circuit is connected to the grid circuit of the oscillator tube for reversing the phase of the fed-back voltages in said feedback circuit, a frequency-determining circuit connected to said oscillator tube consisting of a series arm connected between the plate of the oscillator tube and the grid of said other tube, and a shunt arm connected between the grid of said other tube and its cathode, said series arm comprising a transformer connected to a variable resistance tube of the grid-controlled type and said shunt arm also including a transformer having its effective resistance controlled by a variable resistance tube of the grid-controlled type, and means to impress a signal voltage substantially simultaneously on the grids of said variable resistance tubes to vary their effective plate resistances and thereby to vary the frequency of oscillations by varying the effective resistance of said frequencydetermining circuit.

ROBERT M. SPRAGUE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,872,364 Trouant Aug. 16, 1932 1,938,749 Lindenblad Dec. 12, 1933 2,024,489 Van der Pol et al- Dec. 17, 1935 2,160,528 Usselman May 30, 1939 2,301,160 Finch Nov. 3, 1942 2,321,269 Artzt June 8, 1943 2,268,872 Hewlett Jan. 6,1942 2,173,427 Scott Sept. 19, 1939 OTHER REFERENCES Proceedings of the I. R. E., vol. 31, No. 1, J anuary 1943, pp. 22-25, A Frequency-Modulated Resistance-Capacitance Oscillator, by C. K. Chang, 

